Are people with chronic pain more diverse than we think? An investigation of ergodicity

AbstractThis study investigates whether data from people with chronic pain conditions, including endometriosis (n = 58) and fibromyalgia (n = 58), exhibit ergodicity, a necessary condition for the generalizability of group-based research findings to individual cases. The study focused on frequently assessed variables in chronic pain, including pain intensity, pain interference, depressive symptoms, psychological flexibility, and pain catastrophizing. These data were collected twice daily for 42 days from each participant and then analyzed in two ways: first, as separate cross-sectional studies using the time points as the separate datasets, and second, as individual within-person studies using each individual person’s time series. Results from the two approaches were then compared, including summary means and correlational analyses. For both the endometriosis and fibromyalgia samples, the between-person data showed substantially less variability, and other differences, compared to within-person data. This was evident in both the summary means and correlational analyses. Between-person correlations, for example, were relatively restricted in range, while within-person correlations varied widely, and were not well represented by the between-person estimates. After removing temporal dependency, the strength of within-person correlations decreased, further highlighting the differences in results from the two analytical approaches. The significant findings of this study have potentially profound implications for the field of chronic pain research. They not only underscore the limitations of relying solely on nomothetic methods for understanding individual pain experiences but also strongly advocate for a shift toward inclusion of more idiographic approaches. This shift could lead to more personalized and effective treatment strategies by better capturing the dynamic, highly individual, and heterogeneous nature of chronic pain. To put it bluntly, these results question the assumption that aggregated data collected from groups can accurately represent individual experiences in chronic pain. Keywords: chronic pain, endometriosis, fibromyalgia, group-to-individual generalizability, individual variability